1. Field of the Invention
The present invention relates to a method and apparatus for cooling molded plastic articles after the molding operation is finished. In particular, the present invention relates to method and apparatus for an injection molding machine equipped with a post mold cooling (“PMC”) device mounted on a moving platen that cooperates with a multi-position robot take out plate to both cool the interior of the parts and to (preferably) selectively unload some of the molded part carriers on the multi-position take out plate. The method and apparatus are particularly well suited for cooling injection molded thermoplastic polyester polymer materials, such as polyethylene terephthalate (“PET”) preforms.
2. Related Art
A variety of post mold cooling methods are currently employed on injection molding machines to optimize the cooling of freshly molded plastic parts. Such methods include conductively cooling the parts while they are still inside the mold cavities, blowing air on the exteriors of the molded parts after they are extracted from the mold, and blowing air into the interiors of the molded parts. Some parts (for example plastic preforms) are typically injection-molded using PET resin, and can have wall thicknesses varying from about 2.00 mm to greater than 4.00 mm, and require extended cooling periods to solidify into substantially defect-free parts. Heavy walled parts (such as those made from a material that has a high resistance to thermal heat transfer, like plastic resin) can exhibit “reheating” phenomena that can produce defective parts after they have been ejected from the mold.
In the case of PET preforms, some manufacturing defects are:    Crystallinity: The resin recrystallizes due to the elevated temperature of the core resin not cooling quickly enough. The white appearance of the crystals impairs the clarity of the final product and provides an area of potential weakness in a resultant blown product.    Surface blemishes: The ejected performs, initially having solidified surfaces are reheated by the core material which causes the surface to soften and be easily marred. Sometimes this surface reheating can be severe enough to cause touching parts to weld together.    Geometric inaccuracies: Handling partly-cooled performs or attempting to further cool them in devices that do not maintain their geometric shape while their surfaces are reheated can cause the preform's round diameter to become oval shaped or the smooth surface to become wrinkled or non-linear.
The above-noted problems could be alleviated somewhat by extending the cooling time of the injection molded performs in their mold. However, this will cause the injection molding cycle to be lengthened, typically 25 seconds or longer, wherein the majority of this time would be used solely for cooling purposes. In an effort to improve the production efficiency of this process, several techniques are employed to perform a post mold cooling function, wherein partially cooled preforms are ejected from the injection mold after an initially cooled surface skin has formed to allow the part to be ejected without deformation. The partially cooled preforms are then handed off to a downstream device that continues to hold the preform while removing the remaining heat so that the preform can subsequently be handled without damage. Typically, the preform surface temperature needs to be lowered to about 70° C. to ensure safe handling.
The early ejection of partially cooled preforms releases the injection molding equipment earlier in the molding cycle, thereby significantly improving the production efficiency of the equipment. Injection molding cycle times typically were halved from 25 seconds to about 12 seconds or less (in some instances) depending on the preform design being molded.
Some examples of post mold cooling technology are shown in U.S. Pat. Nos.: 3,804,568; 4,729,732; 4,836,767; Re. 33,237; 5,447,426; and 6,171,541.
U.S. Pat. No. Re. 33,237 discloses a robotically-controlled multi-position take out plate for removing partially cooled injection molded parts from the core side of an injection mold. The parts are ejected from the mold directly into cooled carriers, as disclosed in U.S. Pat. No. 4,729,732, and transported by the robot to an outboard position where some of the parts are ejected onto a conveyor. The plate has multiple sets of carriers, each set being sufficient in number to hold one part from each of the cores of the multi-cavity mold. There are multiple sets of carriers on the plate so that multiple sets of molded parts can be held and cooled, the set that is ejected being the set that has been cooling the longest in the tubes of the plate. However, these patent documents do not disclose cooling the interior of the parts. Moreover, the disclosed method of ejecting the parts relies on the termination of a vacuum that is holding the parts in the carriers, thereby allowing gravity to cause the parts to fall out when the take out plate has been rotated 90 degrees to a discharge position.
U.S. Pat. No. 6,171,541 discloses inserting a cooling pin (CoolJet™) into the interior of partially cooled part to discharge a cooling fluid therein to assist cooling. Also disclosed therein is a procedure to apply a vacuum through the same cooling pin to cause the part to remain attached to the pin when it is moved away from the carrier holding the part, thereby removing the part from the carrier. The pins, mounted to a frame, are then rotated 90 degrees to a discharge position and the vacuum terminated to allow the parts to fall off the pins. However, there is no disclosure of mounting the frame and pins onto a moving platen to utilize the motion of the moving platen to insert and retract the pins with respect to the parts.
U.S. Pat. No. 4,836,767 discloses a rotatable table mounted on the moving platen on which are mounted two core sets for the mold. While one core set is in the closed mold position for injection molding parts, the other is positioned outboard for ejecting the parts into cooled carriers that are mounted on an indexable, four-sided carousel that is mounted to the stationary platen of the machine. Four sets of molded parts can be carried on the carousel allowing an extended cooling time to be performed. The parts remain on the cores for one additional cycle time sequence that provides a small extension of cooling time of the interior of the parts before they are transferred to the carousel. However, there is no disclosure of repeated or multiple cooling of the parts' interiors.
U.S. Pat. No. 3,804,568 discloses a robot mounted to the moving platen of an injection molding machine, wherein the robot drives a take out plate into and out of the open mold area to remove ejected parts. A second transfer plate then unloads the take out plate while it is in the outboard position. The motion of the moving platen is used, via cams and linkages, to actuate the take out plate vertical motion and to synchronize it mechanically so that there is no risk of collision with the mold during its operation. However, there is no disclosure of part cooling, either exterior or interior, while the parts are being transported by either plate.
U.S. Pat. No. 5,354,194 discloses a molded part removal unit mounted to the side of the fixed platen. However, there is no disclosure of any cooling treatment.
An earlier Husky preform molding system used a robot with a single position take out plate with carriers to unload PET preforms. The robot was mounted on the stationary platen and moved the take out plate vertically. In the outboard position, above the mold, a vacuum tube carrier of a transfer plate was aligned with the carriers and removed the molded parts therefrom by application of vacuum to their interiors. The transfer plate moved to a second outboard position at the non-operator side of the machine and rotated to allow the parts to drop from the tubes when the vacuum was terminated. However, there was no blowing or cooling of the interior of the parts during their handling.
With reference to FIGS. 1–4, top plan views of an injection molding machine 10 are shown comprising, an injection unit 11, a clamp unit 12, a robot unit 13, and a CoolJet™ unit 14. Also included is an injection mold comprising two halves: (i) the cavity half 15, containing mold cavities 19, attached to the stationary platen 16 of the machine 10; and (ii) the core half 17 which is attached to the moving platen 18 of the machine 10.
The robot unit 13 is mounted atop the stationary platen 16 and includes a horizontal “Z” beam 20 that projects to the non-operator side of the machine and upon which rides a carriage 21, moved along the beam by (typically) a servo-electric driven belt drive (not shown). Vertical “Y” beam 22 is attached to the carriage 21 and this supports the multi-position take out plate 23 upon which are mounted multiple sets of carriers 24 that may be cooled for transporting multiple molded shots of parts ejected from the mold from an inboard (loading) position, as shown in FIG. 1, to an outboard position as shown in FIGS. 2–4 inclusive.
The transfer device 14 includes a plate 25 upon which are mounted multiple transfer pins 26, one for each carrier 24 on the multi-position take out plate 23. The plate 25 is supported on slides 27 and can be moved toward and away from the carriers 24, when in their outboard position, by cylinder 28.
In operation, one shot of molded parts is transferred into the carriers 24 when the mold is open and the multi-position take off plate 23 is positioned such that empty carriers are aligned with parts on the mold cores 29. In the example shown in FIG. 1, a 48-cavity mold is transferring 48 parts into 48 carriers on a 3 position take off plate 23. The multi-position take off plate 23 is then moved to its outboard position by the robot 13, as shown in FIG. 2. The mold is then closed and clamped for the next molding cycle. Meanwhile, the transfer device 14 activates a cylinder 28 to move the plate 25 and its transfer pins 26 so as to enter the parts held in the carriers 24. This engaged position is shown in FIG. 3.
Just before the molding cycle ends, the transfer pins 26 are extracted from the parts, and the robot 13 causes the multi-position take off plate 23 to rotate 90 degrees (as shown in FIG. 4) by means of a servo motor on the end of an arm 22, or alternatively a crank and cylinder arrangement (not shown). The respective vacuums holding the parts in the carriers 24 are selectively shut off in the order of the parts that have been held in the carriers the longest, in this example for three molding cycles. These parts fall out of the carriers onto a conveyor beneath (not shown). The remaining parts continue to be held in their carriers by vacuum. The multi-position take off plate 23 is then returned to the vertical orientation ready for entry into the open mold area to pick up the next shot of molded parts in the recently vacated carriers 24.
The injection molding machine described above therefore unloads the molded parts into the multi-position take off plate 23 by positioning the plate at various inboard locations to fill the most recently vacated carriers, and then moves them to one outboard position aligned with the transfer device. This outboard position is the same in all cases, where all the parts are dealt with by the transfer device. Thus, each part receives the same treatment the same number of times as there are sets of carriers 24 on the multi-position take off plate 23, in this example three times.
A number of disadvantages are present in the injection molding machine configuration described with respect to FIGS. 1–4. First, the multi-position take off plate 23 is heavy. In larger systems such as those with 432 carriers (to operate with a mold having 144 cavities), the plate can weigh in excess of several hundred kilograms (Kg), as the weight includes not only the structure of the plate and carriers themselves but also the weight of multiple shots of parts plus the weight of any cooling fluid in the plates and carriers, typically water. The effect of this heavy weight when mounted on the end of a cantilevered Y beam 22 (which itself is movably mounted on a cantilevered Z beam 20) is to cause difficulty in maintaining alignment of the carriers 24 with the mold cores and the carriers 26 after the take of plate 23 has moved quickly from the inboard position to the outboard position, and vise versa. The inertia of the plate can cause it to vibrate when quickly being brought to rest in one of its stationary positions, and cycle time can be lost in waiting for motion oscillations to damp out sufficiently before attempting a part transfer or cooling tube insertion.
A second disadvantage in the injection molding machine configuration described above is that when unloading the carriers 24, the entire multi-position take off plate 23 must be rotated 90 degrees and back again quickly. Again, because of the weight and inertia involved, a high-performance, high-cost actuation device must be used if the rotation is not to take too long, since the time taken for this motion is time unavailable for CoolJet™ treatment.
Furthermore, since the transfer device 14 engages each part in each carrier on every cycle, all the parts receive multiple applications of the same treatment. The ability to provide different treatments to the parts in these multiple events is not possible.
A third disadvantage is that the time available for treatment to be applied by the CoolJet™ device is reduced by the time it takes for the multi-position take off plate 23 to rotate to the horizontal position, eject selected parts and rotate back to the vertical position.